A major source of extremely low frequency electromagnetic variations in the ocean is caused by the motion of the highly conductive water through the earth's magnetic field. These hydrodynamic variations affect magnetic and electric field sensors in the frequency range below 1 Hz. One of the first studies of these fields was performed by Longuet-Higgins et al., whose investigations were concerned with electric fields induced by the steady motion of seawater. See, e.g., Longuet-Higgins, M. S., M. E. Stem, and H. Stommel, “The Electric Field Induced by Ocean Currents and Waves, With Applications to the Method of Towed Electrodes,” Papers in Physical Oceanography and Meteorology XIII, 1, Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1954, incorporated herein by reference. Surface waves, internal waves, solitary waves, tides, and ocean currents all produce observable magnetic and electric fields. Larson and Sanford showed that the Florida Current volume transport could be determined from cable voltage measurements of an undersea cable. See, e.g., Larsen, J. C. and T. B. Sanford, “Florida Current Volume Transports from Voltage Measurements,” Science, 227, 302-304, 1985, incorporated herein by reference.
A long period electromagnetic response of internal waves has been observed with magnetic sensors in the deep ocean, but there is little information about the character of the electric and magnetic field generated in shallow water areas. Deep water models for internal wave-induced magnetic spectra indicate that the amplitude increases with decreasing frequency.
Internal wave-induced magnetic fields for a two-layered ocean model were treated by Beal and Weaver. See, e.g., Beal, H. T. and Weaver, J. T., “Calculations of Magnetic Variations Induced by Internal Ocean Waves,” J. Geophys. Res., 75, no. 33, 1970, incorporated herein by reference. Podney followed with a more comprehensive treatment of internal waves for an exponentially stratified ocean with a horizontally uniform Brunt-Vaisala frequency profile. See, e.g., Podney, Walter, “Electromagnetic Fields Generated by Ocean Waves”, J. Geophys. Res., 80, No. 21, 1975, incorporated herein by reference. Wasylkiwskyj used a similar approach to derive solutions for the case where the Vaisala frequency profile decreases exponentially in a manner analogous to that used by Garrett and Munk (1972). See, e.g., Wasylkiwskyj, W., “Electromagnetic Fields Induced by Ocean Currents,” IDA Paper P-1399, IDA: Arlington, Va., 1979, incorporated herein by reference, and Garrett, C. and W. Munk, “Space-Time Scales of Internal Waves,” J. Geophys. Fluid Dynamics, 2, 225-264, 1972, incorporated herein by reference. The solutions derived by Wasylkiwskyj are solved for the case of an airborne sensor moving over the ocean surface. Later on Petersen and Poehls used Podney's formulation combined with the Garrett and Munk model to generate a spectral estimate of the magnetic induction. See, e.g., Petersen, R. A. and K. A. Poehls, “Model Spectrum of Magnetic Induction Caused by Ambient Internal Waves,” J. Geophys. Res., 87, No. C1, 433-440, 1982, incorporated herein by reference. Chave derived a somewhat more general solution for internal waves that used the Garrett and Munk wave spectra. See, e.g., Chave, A. D., “On the Electromagnetic Field Induced by Ocean Internal Waves,” J. Geophys. Res., 89, No. C6, 10519-10528, 1984, incorporated herein by reference. His derivation is applicable both within the ocean and on the ocean bottom and includes the effects of self and mutual induction, which should improve the model solution at low frequencies on the seafloor. He has also computed the results for a theoretical solitary internal wave. See e.g., Chave, A. D., “The Magnetic Effects of Shallow Water Internal Solutions,” Scripps Institute of Oceanography, Reference 86-7, 1986, incorporated herein by reference.